|Series||Swallows and Amazons|
|Media type||Print (Hardcover & Paperback)|
|Preceded by||We Didn't Mean to Go to Sea|
|Followed by||The Big Six|
This book is set in and around Hamford Water in Essex, close to the resort town of Walton-on-the-Naze. It brings the Swallows and the Amazons together and introduces a new group of characters, the Eels and the Mastodon. Ransome used to sail to Hamford Water, an area of tidal salt marshes and low-lying islands, in his yacht Nancy Blackett. He set the book here to offer his characters new opportunities to explore and make maps in a different landscape.
The Swallows intend to sail in the Goblin to Hamford Water and camp with their father, but he is called away on naval business. Instead he maroons them with a small dinghy on an island. Before he leaves, Father gives them an outline map of the area they decide to call Secret Water and suggests they survey and chart the area before he returns to pick them up. For a surprise, he has arranged for the Amazons to come down from the Lake District and join them with another dinghy.
They see some mysterious footprints which turn out to belong to the Mastodon, a local boy. He mistakes them for the Eels, another family who camp in the area regularly. Later the Eels arrive and are initially hostile before they settle down for a friendly war.
It seems that due to the distractions of war and being cut off by the tides, the chart will not be completed. However early in the last morning two separate groups of children complete it.